1. Field of the Invention
The present invention relates to a semiconductor device comprising a high resistance element formed from polysilicon and particularly superior in terms of a relative accuracy in comparison to any other high resistance element formed from polysilicon and a method of manufacturing the semiconductor device.
2. Description of the Related Art
Today, an analog circuit, a typical example of which is a system power supply circuit, is characterized in its large-scale circuit configuration comprising a plurality of mixed analog circuits such as a variable amplifier, a regulator and a charging voltage control circuit. In these exemplified circuits, wherein resistance elements constitute the circuits, a chip size tends to be larger along with the increasingly larger circuit scale because a large number of resistance elements are provided and resistance values thereof are high. In the recent trend, however, the chip size is reduced in order to comply with the reduction of devices in size (a typical example is a mobile telephone). Further, there are different reference potentials in different circuit blocks in these analog circuits. Therefore, it is necessary for the resistance elements constituting these circuits to be independent from a semiconductor substrate potential in order to obtain stable circuit characteristics. As a result, high resistance elements formed from polysilicon, in place of diffused resistors, are now often provided in these analog circuits.
However, in the high resistance element formed from polysilicon (for example, silicon nitride film formed thereon, which is used as a passivation film, deposited by means of the plasma CVD method), hydrogen is generated from silane gas used as a material when the nitride film is deposited, and hydrogen is thereby included in the nitride film itself in a large percentage. Therefore, when hydrogen present in the film diffuses in polysilicon (resistor), the problem that the resistance values unfavorably become variable arises.
In order to solve the foregoing problem, the high resistance element formed from polysilicon, and capable of preventing the diffusion of hydrogen in polysilicon during a manufacturing process and maintaining a high resistance value in a stable manner was proposed (for example, see Patent No. 3195828 of the Japanese Patent Gazette).
In polysilicon including a number of grain boundaries, dangling bonds are easily generated, which leads to the generation of a large number of electrically active defects in polysilicon. It is known that these dangling bonds are stabilized by hydrogen. Hydrogen has a very high reactivity and can be speedily diffused in a solid. In the presence of the film which includes a large amount of hydrogen, such as the plasma silicon film, on the high resistance element, therefore, hydrogen included in the film diffuses in polysilicon, and the dangling bonds are thereby terminated. As a result, the conductance of the polysilicon is reduced.
In the high resistance element formed from polysilicon, wherein an introduced impurity concentration is low, there are a large number of dangling bonds in polysilicon. Of the resistances formed from polysilicon, a low resistance element, which is formed from polysilicon and characterized in its high introduced impurity concentration, includes less dangling bonds in comparison to the high resistance element. The variability of the resistance value resulting from polysilicon depends on the variation of the dangling bonds included in polysilicon. Therefore, the variability of the resistance value, which is generated by hydrogen of the resistance element formed from polysilicon, hardly thereby affects the low resistance element, while the reriability of the resistance value largely thereby affects the high resistance element.